Calibrating passive LC sensor

ABSTRACT

A flexible, passive pressure sensor includes three LC tank circuits. The first LC tank circuit is a pressure sensing LC tank circuit, having a capacitance that varies in response to changes in environmental pressure. The second and third LC tank circuits are reference LC tank circuits, having capacitances that are relatively constant over changes in environmental pressure. A measurement tool measures the resonant frequencies of the three LC tank circuits and then computes a pressure measurement that accounts for changes in resonant frequencies in the LC tank circuits due to environmental effects and deforming.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.62/380,203, filed Aug. 26, 2016, and titled CALIBRATING PASSIVE LCSENSOR. The contents of that application (including the Appendix) areincorporated herein by reference for all purposes.

FIELD

Embodiments of the present disclosure generally relate to improvedpassive LC sensors for medical devices. More specifically, embodimentsof the present disclosure relate to sensors and techniques for moreprecisely measuring and monitoring pressure within a blood vessel.

BACKGROUND

Measuring blood pressure is an important diagnostic tool in many medicaltreatments, especially when treating vascular maladies. For example,aneurysms are often treated by implanting a stent-graft within theaneurysm pocket. Monitoring blood pressure at the stent-graft can beimportant in tracking patient health and treatment effectiveness.Various pressure sensors have been proposed for monitoring bloodpressure within a vessel, including capacitive pressure sensors. Amongthem, thin flexible inductive-capacitive (LC) pressure sensors havegreat potential to integrate within a graft or stent. However, these LCsensors may have significant variances caused by interactions withsurrounding tissue and/or variances caused by deforming circuitcomponents.

SUMMARY

Embodiments of the present disclosure provide improved measurementcorrection techniques and an improved passive sensor by integrating twoor more reference LC tank circuits, which can compensate forenvironmental tissue dielectric effects and/or the effects causedphysical bending and deforming on the sensor's measurements.

According to one example, a passive inductor-capacitor pressure sensorincludes three LC tank circuits. The first LC tank circuit is a pressuresensing LC tank circuit, having a capacitance that varies in response tochanges in environmental pressure and a resonant frequency that dependson the inductance and the capacitance of the LC tank circuit. The secondand third LC tank circuits are reference LC tank circuits, havingcapacitances that are relatively constant over changes in environmentalpressure, for example, less than 0.05% change from 500 mmHg to 1000 mmHg(from high altitude to below sea level) and resonant frequencies thatdepend on the inductances and the capacitances of those LC tankcircuits.

In one variant of that example, the first LC tank circuit is locatedbetween the second and third LC tank circuits. In another variant ofthat example, the first LC tank circuit is located at one end of thepassive inductor-capacitor pressure sensor. In a third variant, thethree LC tank circuits are placed in close proximity to each other sothat the interactions with the environment and the bending and/ordeforming components are approximately equal across all three LC tankcircuits. In one variant of that example, the pressure sensitive LC tankcircuit includes a pressure sensitive dielectric medium or ahermetically sealed cavity for pressure sensing.

In another variant of that example, the second and third LC tankcircuits have inductive coil structures and capacitive structuressubstantially similar or identical to that of the first LC tank circuit,though with two different pressure insensitive dielectric media, so thatdielectric properties of the surrounding media or tissue induce anequivalent parasitic capacitance to all the LC tank circuits and so thatmechanical deformation of the inductive coil structures induces anequivalent inductance change to all the LC tank circuits, with the threeLC tank circuits having different resonant frequencies that areseparated enough to be distinguished by an external sensor reader.

In another example, a method for compensating for shifts in resonantfrequency due to dielectric properties of a surrounding medium andnon-pressure related mechanical deformation in an inductor-capacitorpressure sensor includes: measuring the resonant frequency of apressure-sensing LC tank circuit; measuring the resonant frequency of afirst reference LC tank circuit; and measuring the resonant frequency ofa second reference LC tank circuit. This exemplary method furtherincludes determining a corrected capacitance of the pressure-sensing LCtank circuit utilizing the resonant frequencies of the first and secondreference LC tank circuits using the following equation:

$C_{1V} = {{\frac{m_{1} - m_{2}}{m_{3} - m_{2}}\left( {C_{3} - C_{2}} \right)} + C_{2}}$where C_(1V) is the corrected capacitance, C₂ is the capacitance of thefirst reference LC tank circuit, and C₃ is the capacitance of the secondreference LC tank circuit. In that equation, m₁ is defined as

$\left( \frac{1}{2\pi f_{1}} \right)^{2}$(where f₁ is the resonant frequency of the pressure-sensing LC tankcircuit), m₂ is defined as

$\left( \frac{1}{2\pi f_{2}} \right)^{2}$(where f₂ is the resonant frequency of the first reference LC tankcircuit), and m₃ is defined as

$\left( \frac{1}{2\pi f_{3}} \right)^{2}$(where f₃ is the resonant frequency of the second reference LC tankcircuit).

This exemplary method further includes determining a corrected pressure(P) from the corrected C_(1V) calculation based on the relationshipbetween C_(1V) and P.

In one variant of that example, the pressure-sensing LC tank circuit ispositioned between the first and second reference LC tank circuits. Inanother variant of that example, the pressure-sensing LC tank circuit islocated at one end of the passive inductor-capacitor pressure sensor.

In one variant of that example, determining the corrected capacitance ofthe pressure-sensing LC tank circuit includes accounting for changes inresonant frequencies caused by environmental dielectric propertiesand/or mechanical deformation.

While multiple embodiments are disclosed, still other embodiments of thepresent invention will become apparent to those skilled in the art fromthe following detailed description, which shows and describesillustrative embodiments of the invention. Accordingly, the drawings anddetailed description are to be regarded as illustrative in nature andnot restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a diagram of an exemplary LC tank circuit and aportion of an exemplary circuit for a sensor reader, according toembodiments of the present disclosure.

FIG. 2 illustrates an exemplary pressure sensor coupled to a stent-graftas well as an exemplary measuring tool, according to embodiments of thepresent disclosure.

FIG. 3 illustrates an exemplary pressure sensor having an LC tankcircuit with a planar antenna and said pressure sensor coupled to astent-graft, according to embodiments of the present disclosure.

FIG. 4 illustrates an exemplary pressure sensor having an LC tankcircuit with a cylindrical antenna and that pressure sensor coupled to astent-graft, according to embodiments of the present disclosure.

FIG. 5 illustrates a pressure sensor having three exemplary LC tankcircuits with planar antennas and said pressure sensor coupled to astent-graft, according to embodiments of the present disclosure.

FIG. 6 illustrates a pressure sensor having three exemplary LC tankcircuits with cylindrical antennas, according to embodiments of thepresent disclosure.

FIG. 7 illustrates steps for measuring blood pressure within a vesselusing a pressure sensor with three LC tank circuits, according toembodiments of the present disclosure.

FIG. 8 illustrates a portion of a stent graft to which a pressure sensoris coupled and depicts components of three LC tank circuits that formthe pressure sensor, according to embodiments of the present disclosure.

DETAILED DESCRIPTION

According to some embodiments, FIG. 1 shows an electrical equivalentresonant circuit 1 of a passive LC sensor with an equivalent capacitor C(reference number 7) and inductor L (reference number 5), and anequivalent electrical circuit 3 of a sensor reader, where the readerantenna 13 wirelessly measures the resonant frequency of circuit 1 bymagnetic induction between the sensor antenna (e.g., the inductor 5) andreader antenna 13. The electrical equivalent resonant circuit 1 can alsobe referred to as an LC circuit, LC tank, or LC tank circuit, because ofthe voltage 11 that it can hold. The LC tank 1 has a resonant frequencythat depends on the inductance and capacitance provided by the inductor5 and capacitor 7, respectively. If the capacitor 7 is configured tovary its capacitance in response to a change in external pressure, theLC tank circuit can serve as a pressure sensor. As one of skill in theart will readily appreciate, there are a wide variety of electricalcomponents that exhibit capacitive and inductive characteristics andthat can be used in various embodiments discussed herein. As also shownin FIG. 1, the LC tank circuit 1 also includes an equivalent resistor 9that represents energy loss due to RF absorption. In general, a smallerresistance is required to provide a higher quality factor (Q, e.g.,greater than 35) so that the ring-down signal 15 from the sensor canlast long enough for the reader to pick up the resonance signal.

One of the benefits of an LC tank (e.g., LC tank 1 in FIG. 1) is thatcharacteristics of that circuit (e.g., resonant frequency) can bedetermined without needing to include a power source, such as a battery,as part of the circuit. Instead, a reader or monitoring tool (e.g.,equivalent electrical circuit 3 in FIG. 1) can interact wirelessly withthat circuit to detect those characteristics of the LC tank 1. Anotherbenefit of an LC tank (e.g., LC tank 50 or 60 in FIGS. 3-4) is that theLC circuit can be formed from a thin-flexible structure, such that theLC sensor can be integrated within a stent-graft to measure pressure ata site without blocking blood flow through the vessel. These advantagesrender the LC tank as a suitable candidate for a pressure sensor formonitoring blood pressure within a vessel.

For example, if a dielectric material that reacts to external pressureis placed within the capacitor (e.g., capacitor 7 in FIG. 1), a changein blood pressure will cause a change in capacitance in the LC tank,which results in a change in its resonant frequency. For anotherexample, the LC tank may be set up so that the plates of the capacitor 7move in response to external pressures, which will affect thecapacitance and the resonant frequency of the LC tank. Under eitherapproach, if the other characteristics of the LC tank (e.g., inductance)remain relatively constant, the change in the resonant frequency can beused to determine the change in capacitance, which can then be used todetermine a measurement of the blood pressure within the vessel.

These approaches can be seen in FIG. 2, in which a pressure sensor 20uses an LC circuit to monitor blood pressure at a stent-graft 22 placedto treat an aneurism in the body 24. In various embodiments, thepressure sensor may be placed on an outer surface of the stent-graft, aninner surface of the stent-graft, or may be integrated within thestent-graft. Thus, measuring blood pressure at the stent-graft includesmeasuring blood pressure outside of the stent-graft and/or within thestent-graft. The measurement device 28 includes an antenna 30 and areader/display 32. The reader/display 32 can include a processor,memory, and other hardware and/or software needed to measure signalsfrom the antenna 30 and process those signals to determine (and perhapsdisplay) the blood pressure measurements. The measuring device 28 emitsa pulse 34, which causes the pressure sensor to emit a ring-down signal36. The measuring device 28 analyzes the ring-down signal 36 to identifypressure within the stent-graft 22. Additional details regarding theseprocessing steps are provided below.

In some embodiments, the antenna 30 emits RF signals 34 at a variety offrequencies at different times and measures when the pressure sensor 20absorbs those frequencies. In other embodiments, the antenna 30 emitsenergy at a variety of frequencies simultaneously and then detectsenergy 36 emitted from the LC circuit within the pressure sensor 20,which will indicate the resonant frequency of the LC circuit. Asdiscussed above, measurements of the resonant frequency can be used todetermine blood pressure at the pressure sensor.

At the same time, the accuracy of blood pressure measurements with thinflexible LC sensors is limited by several factors that can also affectthe resonant frequency. For example, tissues surrounding the pressuresensor can have a dielectric effect on the LC circuit's antenna coil,which adds a parasitic capacitance into the LC circuit. Because of thetight restrictions on size for these pressure sensors, any protectivelayers added to the LC circuit may often be thin, such that the abilityto shield the LC circuit from the surrounding media may be limited.Thus, accounting for the effect of the surrounding tissue on thecapacitance of the LC circuit can improve the accuracy of the bloodpressure measurement.

The accuracy of the blood pressure measurement may also be affected bychanges to the inductance of the LC circuit's antenna coil (e.g.,inductor 5 in FIG. 1). Referring now to FIG. 3, an LC tank 50 can bebuilt with a planar antenna. However, to conform to the non-planar shapeof a stent-graft 52, LC tank 50 is bent into a corresponding profile.This can change the shape and inductance of the inductor 5. This effectcan be somewhat mitigated by using a cylindrical antenna, as shown withthe LC tank 60 and the stent-graft 62 in FIG. 6. However, there willtypically be additional changes in inductance as the LC tank 60 issecured to the stent-graft 62. Furthermore, after insertion the LC tank50, 60 will frequently and repeatedly bend and change shape in responseto the pulsating blood flow in the vessel caused by heartbeats. Thus,these factors cause blood pressure measurements taken using a singleflexible LC tank to be less accurate.

In some embodiments, a pressure sensor is formed using multiple LC tanksto improve the accuracy of the blood pressure measurements. Thispressure sensor provides many of the characteristics important forimplantation within a blood vessel, since it is foldable, transmitswirelessly, and does not require internal power (e.g., a battery). Inaddition, as discussed below, this sensor enables more precise bloodpressure measurements within the vessel.

Referring to FIG. 5, a pressure sensor 100 includes three LC tanks102-106, formed with planar antennas, coupled to a stent-graft 110. Thefirst LC tank 102 is pressure sensitive, meaning that its resonantfrequency changes in response to changes in pressure. This can beaccomplished, for example, by including an elastic dielectric materialwithin the capacitor 7. The other two LC tanks 104, 106 are not pressuresensitive, meaning that their resonant frequencies are relativelyunaffected by changes in pressure. However, all three LC tanks areresponsive to physical bending as well as environmental tissuedielectric effect (and other non-pressure factors affecting the resonantfrequency). Thus, the other two LC tanks 104, 106 serve as reference LCtanks that sense tissue dielectric effects and bending and deformingeffects that also occur on the first LC tank 102.

As also shown in FIG. 5, these three LC tanks 102-106 are secured to thestent-graft 110 in close proximity to each other, for example, separatedby a gap of approximately 2-5 millimeters. As a result, factorsaffecting inductance and factors affecting capacitance (other thanpressure) will be the same or substantially similar across the three LCtanks (102-106).

FIG. 6 illustrates a pressure sensor 200 with three LC tanks 202-206formed with cylindrical antennas. FIG. 6 further shows their relativepositioning as if they were coupled to a stent-graft. The first LC tank202 is pressure sensitive, meaning that its resonant frequency changesin response to changes in pressure. This can be accomplished, forexample, by including an elastic dielectric material or vacuum cavitywithin the capacitor 7. The other two LC tanks 204, 206 are not pressuresensitive, meaning that their resonant frequencies are unaffected bychanges in pressure. However, all three LC tanks will be responsive tophysical changes as well as environmental tissue dielectric effect (andother non-pressure factors affecting the resonant frequency). Thus, theother two LC tanks 204, 206 serve as reference LC tanks that sensetissue dielectric effects and bending and deforming effects that alsooccur on the first LC tank 202.

In other words, in both FIGS. 5 and 6, the first LC tank (102, 202) usesa variable or pressure-sensitive capacitor whose capacitance (C_(1V))changes in response to changes in environmental pressures (e.g., changesin blood pressure). The other two LC tanks (104, 106, 204, 206) havecapacitors whose capacitance (C₂, C₃, respectively) do not change inresponse to changes in environmental pressures. These three capacitorsmay be configured with different dielectric media to have threedifferent capacitance values such that the three LC tanks have differentresonant frequencies that can be readily distinguished by an externalreader. With these additional reference measurements, the measurementsof the first LC tank (102, 202) can be corrected to account for tissuedielectric effects as well as bending and deforming effects using themeasurements of the other two LC tanks (104, 106, 204, 206).

In addition, these three LC tanks are constructed to have equivalent orsubstantially equivalent inductance (L), and experience equivalent orsubstantially equivalent parasite capacitance (C_(par)) and equivalentor substantially equivalent substrate and environmental capacitance andloss (C_(sub)). For example, in some embodiments all three LC tanks havesimilar inductive coils or even substantially identical inductive coils.As a result, any bending of the pressure sensor causes equivalent orsubstantially equivalent inductance change (ΔL) in all three LC tanks.

A measurement device (e.g., measurement device 28 in FIG. 2) can measurethe resonant frequencies of the three LC tanks (102-106 or 202-206), andcan process those measured frequencies in a manner that accounts forchanges in inductance and changes in capacitance due to factors otherthan pressure. The three capacitors C1, C2, and C3 have differentcapacitance values, so that the corresponding resonant frequencies ofthree LC tanks are different enough to be distinguished. Additionaldetails regarding these processing steps are provided below. Thus, usinga pressure sensor with these three LC tank circuits and using theseprocessing steps improves the precision of blood pressure measurements.

In an LC circuit, the resonant frequency (f) is:f=1/(2π√{square root over (LC)})  (Eq. 1)where L is the inductance of the LC circuit and C is the capacitance ofthe LC circuit. For simplicity, Equation 1 may be rewritten in terms ofa measurement “m”:

$\begin{matrix}{m = {\left( \frac{1}{2\pi f} \right)^{2} = {LC}}} & \left( {{Eq}.\mspace{14mu} 2} \right)\end{matrix}$

The resonant frequencies of the three LC tanks are measured, resultingin measurements m₁, m₂, and m₃, which relate as follows:

$\begin{matrix}{m_{1} = {\left( \frac{1}{2\pi f_{1}} \right)^{2} = {\left( {L + {\Delta L}} \right)\left( {C_{1V} + C_{par} + C_{sub}} \right)}}} & \left( {{Eq}.\mspace{14mu} 3} \right) \\{m_{2} = {\left( \frac{1}{2\pi f_{2}} \right)^{2} = {\left( {L + {\Delta L}} \right)\left( {C_{2} + C_{par} + C_{sub}} \right)}}} & \left( {{Eq}.\mspace{14mu} 4} \right) \\{m_{3} = {\left( \frac{1}{2\pi f_{3}} \right)^{2} = {\left( {L + {\Delta L}} \right)\left( {C_{3} + C_{par} + C_{sub}} \right)}}} & \left( {{Eq}.\mspace{14mu} 5} \right)\end{matrix}$

In other words, using the three LC tanks in this manner provides threemeasurement (m₁, m₂ and m₃) and equations with three unknowns (C_(1V),C_(par)+C_(sub) and ΔL).

These equations can be manipulated so that two of the three unknownfactors (C_(par)+C_(sub) and ΔL) cancel out. First, two of the equationsare subtracted as shown below:m ₃ −m ₂=(L+ΔL)(C ₃ −C ₂)  (Eq. 6)m ₁ −m ₂=(L+ΔL)(C _(1V) −C ₂)  (Eq. 7)

Then a ratio of these equations are taken:

$\begin{matrix}{\frac{m_{1} - m_{2}}{m_{3} - m_{2}} = \frac{C_{1V} - C_{2}}{C_{3} - C_{2}}} & \left( {{Eq}.\mspace{14mu} 8} \right)\end{matrix}$

Solving this equation for C_(1V) results in the following equation:

$\begin{matrix}{C_{1V} = {{\frac{m_{1} - m_{2}}{m_{3} - m_{2}}\left( {C_{3} - C_{2}} \right)} + C_{2}}} & \left( {{Eq}.\mspace{14mu} 9} \right)\end{matrix}$

In this equation, C_(1V) and m₁ are functions of environmental pressure(P), while m₂, m₃, C₂ and C₃ are independent and will not change withpressure P.

Using the term P₀ to refer to an initial environmental pressure, achange in capacitance (ΔC_(V)) can then be used to determine a change inthe environmental pressure (ΔP). First, we can break up C_(1V) into twoparts:C _(1V) =C _(1V) {P}=C _(1V) {P ₀ +ΔP}=C ₁₀ {P ₀ }+ΔC _(V) {ΔP}  (Eq.10)where brackets are used to indicate that capacitance (e.g., C_(1V)) is afunction of pressure (e.g., P).

Combining Equation 9 and Equation 10 results in the following equation:

$\begin{matrix}{{\frac{{m_{1}\left\{ {P_{0} + {\Delta P}} \right\}} - m_{2}}{m_{3} - m_{2}}\left( {C_{3} - C_{2}} \right)} = {{C_{10}\left\{ P_{0} \right\}} + {\Delta C_{V}\left\{ {\Delta P} \right\}} - C_{2}}} & \left( {{Eq}.\mspace{14mu} 11} \right)\end{matrix}$

If measurements are taken of the three LC tanks at the initial pressureP₀ (i.e., with ΔP=0, ΔC_(V){0}=0) those measurement can be referred toas m₂*, m₁*(P₀), and m₃*, and Equation 11 reduces to:

$\begin{matrix}{{{C_{10}\left\{ P_{0} \right\}} - C_{2}} = {\frac{{m_{1}^{*}\left( P_{0} \right)} - m_{2}^{*}}{m_{3}^{*} - m_{2}^{*}}\left( {C_{3} - C_{2}} \right)}} & \left( {{Eq}.\mspace{14mu} 12} \right)\end{matrix}$

Using Equation 11, Equation 12 can be rewritten as:

$\begin{matrix}{{\Delta C_{V}\left\{ {\Delta P} \right\}} = {\left( {\frac{{m_{1}\left\{ {P_{0} + {\Delta P}} \right\}} - m_{2}}{m_{3} - m_{2}} - \frac{{m_{1}^{*}\left( P_{0} \right)} - m_{2}^{*}}{m_{3}^{*} - m_{2}^{*}}} \right)\left( {C_{3} - C_{2}} \right)}} & \left( {{Eq}.\mspace{14mu} 13} \right)\end{matrix}$

These equations can be further simplified by combining some of the termsinto new variables, as follows:

$\begin{matrix}{{R_{m}^{*}\left( P_{0} \right)} = \frac{{m_{1}^{*}\left\{ P_{0} \right\}} - m_{2}^{*}}{m_{3}^{*} - m_{2}^{*}}} & \left( {{Eq}.\mspace{14mu} 14} \right) \\{{R_{m}\left( {P_{0} + {\Delta P}} \right)} = \frac{{m_{1}\left\{ {P_{0} + {\Delta P}} \right\}} - m_{2}}{m_{3} - m_{2}}} & \left( {{Eq}.\mspace{14mu} 15} \right)\end{matrix}$C _(ref) =C ₃ −C ₂  (Eq. 16)

Using these variables, the earlier equations simplify to:ΔC _(V) {ΔP}=C _(ref)(R _(m) {P ₀ +ΔP}−R _(m) *{P ₀})  (Eq. 17)

In sum, by using this approach, the relationship between ΔC_(V) and ΔPis independent from C_(par)+C_(sub) as well as ΔL. Consequently, acontrolled calibration measurement can be used to derive a curve of ΔP(relative to P₀) vs. ΔC_(V). For example, during one exemplarycalibration technique, a sensor with three LC tanks, which may be builtinto a graft, is placed into an air chamber in which the air pressure ismeasured by a calibrated standard pressure sensor. A set of pressures,including the initial pressure P₀, is set in the air chamber. At eachpressure setting, the resonant frequencies of the three LC tanks arewirelessly detected. Then, using Eq. 17, a calibration curve of ΔP(relative to P₀) vs. ΔC_(V) is computed from the above measurements. Asdiscussed above, this calibration curve is independent from both thesurrounding tissue dielectric effect and the deforming/bending effect.Therefore, only a single calibration for multiple LC tanks as describedabove is needed. This calibration curve of ΔP (relative to P₀) vs.ΔC_(V) is used to identify changes in blood pressure when the device isimplanted.

In addition, the pressure sensor might not be around P₀ when the deviceis implanted. To get to the base point R_(m)*{P₀}, an on-sitecalibration measurement with other means is performed, for example,using a pressure catheter to measure the blood pressure at the site. Thepreviously-derived calibration curve for this sensor will then indicateany pressure change ΔP from the capacitance change ΔC_(V), calculatedfrom the measured resonant frequencies.

An exemplary method 300 for determining blood pressure using a pressuresensor is shown in FIG. 7. As shown in block 302, the method begins withcalibrating the pressure sensor. This includes creating a calibrationcurve for the pressure sensor (e.g., ΔP (relative to P₀) vs. ΔC_(V)) bymeasuring the resonant frequencies of the three LC tanks at a set ofknown pressures. As shown in block 304, the pressure sensor is implantedin the patient. In this step, the flexibility of the LC pressure sensormay facilitate easy implantation. For example, the pressure sensor maybe coupled to a stent-graft in its expanded state. The stent-graft andpressure sensor may be then crushed and/or folded for insertion andsubsequent expansion at the tissue site. In other cases, the LC pressuresensor may be built into an arteriovenous (AV) graft which may besurgically implanted into human body. As shown in block 306, theresonant frequencies of the three LC tank circuits are measuredwirelessly by an external reader. Using those measurements, the changein capacitance for the pressure-sensitive LC tank circuit is determined,using the equations and techniques discussed above, as shown in block308. Finally, as shown in block 310, the change in blood pressure isdetermined using the change in capacitance for the pressure-sensitive LCtank circuit.

In some embodiments, blocks 306, 308, and 310 are performed by ameasurement device (e.g., 28 in FIG. 2). For example, these steps couldbe automatically performed, in whole or in part, by a processorexecuting instructions stored in a tangible, non-transitory storagemedium, without user interaction. These instructions cause the processorto implement the equations and techniques discussed above.

In some embodiments, methods for using a pressure sensor (e.g., apressure sensor with three LC tanks) includes crushing and/or foldingthe pressure sensor for insertion into a patient's body. The flexiblenature of the LC tank circuits enable the pressure sensor to expand withthe graft stent without degrading.

Referring now to FIG. 8, a stent-graft 500 includes a stent 502 and agraft 504. Coupled to the graft 504 are three LC circuits 506, 508, 510.In other embodiments, those circuits are coupled to the stent 502. LCcircuit 506 includes an inductor 512 and a variable, pressure sensitivecapacitor 514 whose capacitance (C_(1v)) changes in response to externalpressures. LC circuits 508, 510 include inductors 516, 518 andcapacitors 520, 522 whose capacitances do not change in response toexternal pressures.

Various modifications and additions can be made to the exemplaryembodiments discussed without departing from the scope of the presentinvention. For example, while the embodiments described above refer toparticular features, the scope of this invention also includesembodiments having different combinations of features and embodimentsthat do not include all of the above described features.

What is claimed is:
 1. A medical device for compensating for dielectricproperties of a surrounding medium and non-pressure related mechanicaldeformations in an inductor-capacitor pressure sensor, the medicaldevice comprising: a stent-graft; a pressure-sensing inductor-capacitortank circuit arranged on an outer surface of the stent-graft, an innersurface of the stent-graft, or integrated within the stent-graft, thepressure-sensing inductor-capacitor tank circuit including a firstinductor and a first capacitor, the pressure-sensing inductor-capacitortank circuit having a capacitance that varies in response to changes inenvironmental pressure; a first reference inductor-capacitor tankcircuit including a second inductor and a second capacitor, the firstreference inductor-capacitor tank circuit having a capacitance that isrelatively constant over changes in environmental pressure; and a secondreference inductor-capacitor tank circuit including a third inductor anda third capacitor, the second reference inductor-capacitor tank circuithaving a capacitance that is relatively constant over changes inenvironmental pressure, wherein the capacitance of the second referenceinductor-capacitor tank is different than the capacitance of the firstreference inductor-capacitor tank.
 2. The medical device of claim 1,wherein the pressure-sensing inductor-capacitor tank circuit ispositioned between the first and second reference inductor-capacitortank circuits.
 3. The medical device of claim 1, wherein thepressure-sensing inductor-capacitor tank circuit, the first referenceinductor-capacitor tank circuit, and the second referenceinductor-capacitor tank circuit are part of a passive inductor-capacitorpressure sensor, and wherein the pressure-sensing inductor-capacitortank circuit is located at one end of the passive inductor-capacitorpressure sensor.
 4. The medical device of claim 1, wherein thepressure-sensing inductor-capacitor tank circuit, the first and secondreference inductor-capacitor tank circuits and the stent-graft arecompressible for insertion into a circulatory vessel.
 5. The medicaldevice of claim 1, wherein the pressure-sensing inductor-capacitor tankcircuit, the first and second reference inductor-capacitor tank circuitsand the stent-graft are expandable to an expanded state within acirculatory vessel.
 6. The medical device of claim 1, wherein thepressure-sensing inductor-capacitor tank circuit is located within 2-5millimeters of the first reference inductor-capacitor tank circuit. 7.The medical device of claim 6, wherein the pressure-sensinginductor-capacitor tank circuit is located within 2-5 millimeters of thesecond reference inductor-capacitor tank circuit.
 8. The medical deviceof claim 1, wherein the first reference inductor-capacitor tank circuitand the second reference inductor-capacitor tank circuit are configuredto have inductive coils substantially identical to that of thepressure-sensing inductor-capacitor tank circuit, such that dielectricproperties of a surrounding media induce an equivalent parasiticcapacitance to the pressure-sensing inductor-capacitor tank circuit, thefirst reference inductor-capacitor tank circuit, and the secondreference inductor-capacitor tank circuit, and mechanical deformation ofthe inductive coils induces an equivalent inductance change to thepressure-sensing inductor-capacitor tank circuit, the first referenceinductor-capacitor tank circuit, and the second referenceinductor-capacitor tank circuit.
 9. The medical device of claim 1,wherein the pressure-sensing inductor-capacitor tank circuit includes anelastic dielectric material within the first capacitor.
 10. The medicaldevice of claim 1, wherein the pressure-sensing inductor-capacitor tankcircuit is foldable.
 11. The medical device of claim 10, wherein thefirst capacitor of the pressure-sensing inductor-capacitor tank circuitcomprises a vacuum cavity within the first capacitor.
 12. The medicaldevice of claim 1, wherein the pressure-sensing inductor-capacitor tankcircuit is formed from a thin flexible structure and integrated withinthe stent-graft to measure pressure without blocking blood flow througha circulatory vessel.
 13. An implantable device, comprising: apressure-sensing inductor-capacitor tank circuit including a firstinductor and a first capacitor, the pressure-sensing inductor-capacitortank circuit having a capacitance that varies in response to changes inenvironmental pressure; a first reference inductor-capacitor tankcircuit including a second inductor and a second capacitor, the firstreference inductor-capacitor tank circuit having a capacitance that isrelatively constant over changes in environmental pressure; and a secondreference inductor-capacitor tank circuit including a third inductor anda third capacitor, the second reference inductor-capacitor tank circuithaving a capacitance that is relatively constant over changes inenvironmental pressure, wherein the capacitance of the second referenceinductor-capacitor tank is different than the capacitance of the firstreference inductor-capacitor tank.
 14. The implantable device of claim13, wherein the pressure-sensing inductor-capacitor tank circuit ispositioned between the first and second reference inductor-capacitortank circuits.
 15. The implantable device of claim 13, wherein thepressure-sensing inductor-capacitor tank circuit, the first referenceinductor-capacitor tank circuit, and the second referenceinductor-capacitor tank circuit are part of a passive inductor-capacitorpressure sensor, and wherein the pressure-sensing inductor-capacitortank circuit is located at one end of the passive inductor-capacitorpressure sensor.
 16. The implantable device of claim 13, wherein thepressure-sensing inductor-capacitor tank circuit, and the first andsecond reference inductor-capacitor tank circuits are compressible forinsertion into a circulatory vessel.
 17. The implantable device of claim13, wherein the pressure-sensing inductor-capacitor tank circuit, andthe first and second reference inductor-capacitor tank circuits areexpandable to an expanded state within a circulatory vessel.
 18. Theimplantable device of claim 13, wherein the pressure-sensinginductor-capacitor tank circuit is located within 2-5 millimeters of thefirst reference inductor-capacitor tank circuit.
 19. The implantabledevice of claim 18, wherein the pressure-sensing inductor-capacitor tankcircuit is located within 2-5 millimeters of the second referenceinductor-capacitor tank circuit.
 20. The implantable device of claim 13,wherein the first reference inductor-capacitor tank circuit and thesecond reference inductor-capacitor tank circuit are configured to haveinductive coils substantially identical to that of the pressure-sensinginductor-capacitor tank circuit, such that dielectric properties of asurrounding media induce an equivalent parasitic capacitance to thepressure-sensing inductor-capacitor tank circuit, the first referenceinductor-capacitor tank circuit, and the second referenceinductor-capacitor tank circuit, and mechanical deformation of theinductive coils induces an equivalent inductance change to thepressure-sensing inductor-capacitor tank circuit, the first referenceinductor-capacitor tank circuit, and the second referenceinductor-capacitor tank circuit.
 21. The implantable device of claim 13,wherein the pressure-sensing inductor-capacitor tank circuit includes anelastic dielectric material within the first capacitor.
 22. Theimplantable device of claim 13, wherein the pressure-sensinginductor-capacitor tank circuit is foldable.
 23. The implantable deviceof claim 13, wherein the pressure-sensing inductor-capacitor tankcircuit is formed from a thin flexible structure and integrated within astent-graft to measure pressure without blocking blood flow through acirculatory vessel.
 24. The implantable device of claim 23, wherein thefirst capacitor of the pressure-sensing inductor-capacitor tank circuitcomprises a vacuum cavity within the first capacitor.